Sayyidah Aishah (may Allah be pleased with her) says that it was said to me: O Mother of the Believers, can the month of Ramadan be twenty-nine days? She said: I fasted twenty-nine days with the Messenger of Allah (peace and blessings be upon him) more often than thirty.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8201